1. Field of the Invention
A method for allocating echo canceling devices in a telecommunications network having at least two networks connected to one another by network switching nodes having trunk groups leading to other networks, through which networks connected terminal devices can be connected to one another. Devices for canceling echos are provided in the networks. The devices can be requested and, if certain criteria are fulfilled, inserted into existing connections in order to cancel echo signals. The method includes marking selected trunk groups originating from or leading to another network with an echo cancellation mark (ecm) characteristic; and providing available echo canceling devices based upon the ecm characteristic when echo canceling is requested for a connection being set up through one of the selected trunk groups.
2. Discussion of Background Information
As is generally known, echo signals occur in telecommunications networks, which are undesirable when the transmission time of the echo exceeds a certain value and therefore disturbs users currently communicating. The echo signals occur because of unavoidable reflections in 2-wire/4-wire transmission in the hybrid circuit or because of direct acoustic coupling in the subscriber's terminal. In order to reduce or completely eliminate echo signals in connections having relatively long transmission times, network operators provide devices for echo suppression, such as echo suppressors or echo cancelers, in certain switching nodes, which are activated and inserted into the connection when necessary.
Echo suppression logic in the switching nodes uses various criteria to decide whether a device for echo suppression needs to be inserted for a particular connection. These criteria include:                permanently programmed routing data with information about the distance (and therefore the transmission time) between the source and the destination of a connection, whereby the device for echo suppression is inserted when a certain distance is exceeded,        the transmission time of the useful signal, which is dynamically determined when a connection is set up,        information concerning the terminal device/connection (with or without an echo source), i.e., data that give information about whether or not an echo will occur in a given connection, and        information received from other switching nodes involved in setting up a connection about the allocation of devices for echo suppression.        
In this method, the echo suppression logic attempts to activate a device for echo suppression located as near to the source of the echo as possible.
According to ITU (International Telecommunications Union) Rec. Q.115(97), two possibilities for allocating devices for echo suppression are provided, namely, on one hand, the dedicated assignment of devices for echo suppression to transmission paths and/or trunk groups and, on the other hand, the arrangement of several devices for echo suppression in a pool. In both cases a device for echo suppression will be activated when needed by a control signal from the switching nodes, where, as per ITU Rec. Q115(97), the “technical” criteria mentioned above are the controlling factors. Allocating devices for echo suppression requires the network operator to make significant additional investments in his or her network, which is why he will only provide as many of these devices as are absolutely necessary. In view of the constantly increasing percentage of non-voice services, the allocation of devices for echo suppression in a pool would be the most economical. But the possibility cannot be excluded that network operators who use networks belonging to other network operators to set up connections, intentionally or unintentionally (by sending out information that has been manipulated and is relevant for the echo suppression logic), also use the other network's devices for echo suppression without authorization and can therefore save on investments for devices of this sort in their own network. With the breaking up of the monopolistic structure, the network operators are engaging in increasingly intense competition with one another, which is why the economic factor is increasingly important to the network operators. Therefore, the unauthorized use by other network operators of a network operator's resources, such as the devices for echo suppression, must be prevented.